Machine automation frequently involves measuring small distances extremely accurately. Wire Bonding is one such application with some challenging requirements.
A ‚Äònormal‚Äô wire bond consists of gold wire bonded to a substrate (PC Board) and an integrated circuit. Wire size varies between eighteen and fifty microns, with twenty-five microns being the ‚Äòstandard‚Äô; pad sizes are typically between seventy-five and one hundred microns wide. Space between two pads is generally one hundred microns. The width of the actual bond is generally 1.2 to 2.5 times the diameter of the wire. With tolerances that close, the movement of the stage has to be monitored extremely accurately to ensure proper placement.
The wire bonds are designed to be adjacent to each other and maintain the same center-to-center pitch as they pass from die-to-die. We call this a coplanar wire bond structure because the wire bonds are always parallel to each other and on the same plane along all cross-sections of the structure. By always using the center wire as the signal and the two outer wires as the grounds (or return), a controlled impedance structure is created. -¬†Samuel Harkness, Jeffrey Meirhofer, and Brock J. LaMeres — Department of Electrical and Computer Engineering Montana State University, Bozeman, MT¬†
Two hot (300C) surfaces needed to be checked and confirmed to be coplanar to within a few microns prior to bonding.
MTI Instruments High Temperature Capacitance Probes
work in hot environments and have accuracies rivaling that of laser interferometers. A special custom probe was designed with approximately 0.5 mm range that could withstand 300C. The probe was mated to a MTI AS-563-PA
probe amplifier board which was integrated into their wire bonder. The accuracy achieved in the hot environment was greater than required and the program was very successful.